High intensity lights are needed for applications such as navigation beacons. For example, navigation lamps must be capable of meeting the 20,000 cd requirements for the FAA (US Federal Aviation Authority which sets requirements for airfield and obstruction lighting in the United States and North America) L865-L864 standard and the ICAO (International Civil Aviation Organization which sets requirements for airfield and obstruction lighting for Europe and most international regions outside North America) Medium Intensity Navigation Lights. In the past, lamps have used conventional strobe lights. However, such lights are energy and maintenance intensive. Recently, lamps have been fabricated using light emitting diodes (LEDs). LEDs create unique requirements in order to be commercially viable in terms of size, weight, price, and cost of ownership compared to conventional strobe lights.
Requirements for high intensity lights used for airfield and obstruction lighting are expressed in azimuth, elevation, horizon intensity and radials among other terms. Azimuth is an angular measurement in spherical coordinates measured in degrees. Azimuth angles are perpendicular to a center axis measured in the horizon plane. Generally, airfield and obstruction lighting are omni-directional, meaning they emit light over 360° of azimuth. Elevation is an angular measurement in spherical coordinates measured in degrees. Elevation angles are measured plus and minus from horizon. Positive elevation angles are angles above horizon, and negative elevation angles are angles below horizon. The horizon is a plane at an elevation angle of 0° which is parallel to the earth and perpendicular to the force of gravity. The location of the horizon plane is at the center of a light engine. Intensity or beam intensity is the luminous power emitted by a source in a particular direction, expressed in Candela, abbreviated as (cd). Radial is a plane slice taken perpendicular to any azimuth angle and is typically used to describe beam intensity at various elevation angles. Photometric specifications require beam intensities to be within defined parameters for every radial over 360° of azimuth.
The FAA and ICAO regulations set stringent requirements for beam characteristics at all angles of rotation (azimuth) for a light engine. For the ICAO standard, lights must have effective (time-averaged) intensity greater than 7500 candela (cd) over a 3° range of tilt (elevation). Lights must also have peak effective intensity of greater than 15,000 at the horizon and peak effective intensity no greater than 25,000 cd at all angles of elevation. An effective intensity at −10° elevation must be no greater than 3% of peak intensity at that azimuth radial. Also a very narrow “window” of effective intensity of 7,500-11,250 cd at −1° of elevation for all angles of rotation must be met.
Similar standards must be met for an ICAO compliant 2,000 cd beacon. For this beacon, ICAO regulations set the stringent requirements for beam characteristics at all angles of rotation. The effective (time-averaged) intensity must be greater than 750 cd over a 3° range of elevation. The peak effective intensity must be greater than 1,500 cd at 0° elevation and no greater than 2,500 cd at all angles of elevation. A very narrow “window” of effective intensity (750-1,125 cd) at −1° of elevation for all angles of azimuth must be met.
It is desirable that light devices meeting ICAO requirements for 20,000 cd lights also meet the requirements of the FAA specifications for L865 and L864 Medium Intensity Obstruction Lights. Such lights must have effective (time-averaged) intensity greater than 7,500 cd over a 3° range of elevation. The peak effective intensity must be greater than 15,000 cd at 0° elevation and no greater than 25,000 cd at all angles of elevation.
Other countries have different, but still stringent, requirements for high intensity obstruction lights. For example, Germany has a 170 cd minimum red obstruction specification which must comply with “Bundesminesterium für Verkehr, Bau-und Wohnungswesen” and as specified in “Nachricthen für Luftfahrer” general administrative regulation for marking and lighting obstacles to air navigation specific to Annex 3: “Specifications for W red lights.”
Similarly, the United Kingdom has a specification for a 2,000 cd red obstruction light. The United Kingdom specification is per “CAP 393 Air Navigation: Order and the Regulations” for lighting of wind turbine generators in UK territorial waters. This specification includes requirements that the angle of the plane of the beam of peak intensity emitted by the light must be elevated 3-4° above the horizontal plane and not more than 45% or less than 20% of the minimum peak intensity must be visible at the horizontal plane. Further, not more than 10% of the minimum peak intensity must be visible at 1.5° or more below the horizontal plane.
In order to achieve the total light intensity required for a light using LEDs compliant with FAA, ICAO and other standards, it is necessary to use a large number of LED light sources. However, it is difficult to create a beam with the desired intensity pattern when directing large numbers of LED sources into few reflectors. Furthermore, smaller and therefore more numerous reflectors are needed to conform to overall size restrictions. These constraints all result in a design with a large number of optical elements comprised of individual LEDs and small reflectors. A final challenge is that even at any one angle of azimuth, it is difficult to achieve an elevation beam pattern which simultaneously satisfies the ICAO requirements for peak (maximum) intensity and also falls within the minimum and maximum intensity “window” at −1°. It is also difficult to achieve the same elevation beam pattern at all angles of azimuth. Since the elevation beam patterns must fall within the required limits at all angles of azimuth, this further compounds the difficulty of meeting the full specifications of the FAA, ICAO and other organizations.
Thus an efficient LED-based based light that meets FAA, ICAO and other standards is desirable. An LED based light design allowing the use of relatively small reflectors and may be scaled to meet different standards is also desirable. An LED based light design that reliably provides uniform light beam output over all angles of azimuth in compliance with such standards also does not exist.